1. Field of the Invention
The present invention relates to a digital image forming apparatus such as a digital printer or a digital copying machine wherein the light intensity for exposing the photoconductor is modulated according to image data to express gradation.
2. Description of the Prior Art
In a digital image forming apparatus wherein an image is formed by changing the light intensity of laser beam in the electrophotographic process, the photoconductor is exposed at a light intensity modulated in correspondence to a multi-level read density data of a document image. In general, the read image density of a document read by a sensor is not proportional to the reproduced image density due to the photosensitive characteristics of the photoconductor or the characteristics of toners. In other words, the gradation characteristic of the reproduced image density level shows a nonlinear characteristic in contrast to the linear characteristic to be realized. Especially, this is a large factor to deteriorate the fidelity of the reproduced image for a half-tone image included in a document.
The gradation characteristic is also affected largely by a technique to convert the read image density level to the light emitting level for exposure. The digital gradation expression techniques with use of the laser intensity modulation includes techniques such as laser intensity modulation, pulse width modulation and area gradation.
In the laser intensity modulation technique, the light intensity of a dot of a laser beam is modulated to express the gradation according to an image data. This technique can have a high resolution and can make the gradation expression smooth. However, as to the high-speed switching performance of laser, the pulse width modulation technique has a problem on the increase in switching speed, while the laser intensity modulation technique is advantageous on this point. Further, the area modulation technique such as a dither method has a bad resolution. Therefore, the laser intensity modulation technique is expected to become one of main digital gradation techniques.
However, the laser intensity modulation technique has following problems: (a) Pitch noises, which is characteristic of the laser exposure, such as the irregularities of the sensitizing of the photoconductor by the charger or of the development are liable to occur in the subscan direction. (b) Noises of image data sent from an image reader are also reproduced faithfully. (c) Because the high gradation can be realized basically, a reproduced image may lack the smoothness as for the impression of a whole image.
The image quality and noises are especially important for the reproduction of a color image because a document often includes a pictorial image.